Telephone system



Original Filed May 12, 1938 8 Sheets-Sheet 2 1&8 2 3 .K N 0 mm -NNU N ommu H X3 31 L L M E 23 hr J w S 2 3 EMU m 8?; $32 =..0|P

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Nov. 10, 1942. R. H. HERRICK TELEPHONE SYSTEI Original Filed lay 12, 1938 8 Sheets-Sheet 6 DHHAHOR ROSWELL H. HE R RICK ATTORNBYfi Original Filed May 12. 1938 8 Sheets-Sheet 8 no 0. h. 25 E 8m 29E 32; 1 .7 .633 Ill 58:15: NS m V 26/ J 21 on: FEE

N6 50 m o 25 R N 3o 3 2.3 NB 0 .5 Ebu 30 23 r M50 N3 30 Ibo :5 owm omw ullllil 12.3 L a \26 E? .2 .36 25 23 :5 F36 INVENI'OR ROSWELL HERRICK ATTORNEY 5.

iii?) Reissued Nov. 10, 1942 TELEPHONE SYSTEM Roswell Harry Herrick, Oak Park, Ill., assignor to Associated Electric Laboratories, Inc., Chicago, IlL, a corporation of Delaware Original No. 2,187,902, dated January 23, 1940,

Serial No. 207,433, May 12, 1938.

Application for reissue August 19, 1940, Serial No. 353,316

44 Claims.

The present invention relates to telephone systems and more particularly to improved telephone apparatus for providing simplified and more direct calling facilities between the telephone substation of an important executive or official and the telephone substations of subordinates who are frequently called from the executive telephone substation.

As explained in Patent No. l,633,166--Clarence E. Lomax, granted June 21, 1927, it sometimes occurs that one party located at a given substation may have occasion to call the telephones of a particular group of other parties at frequent intervals. This is particularly true of an executive who relies upon telephone facilities to secure information from his immediate subordinates. In a situation of this character, the delay incident to the operation of a manual system' or the labor incident to the operation of the calling device in automatic systems may prove burdensome to the party initiating the calls. In order to simplify the calling operation, the improved direct call system disclosed in the above-mentioned patent was developed. This system is so arranged that calls to a number of telephones may be initiated at the special substation either by using a conventional dial operated calling device or by the simple expedient of selectively actuating calling keys individually corresponding to the telephones which are frequently called. In the disclosed arrangement, a single line extends to the executive substation and only one set of transmitting and receiving means is provided for use in communicating over the line. Hence, only a single call can be handled at any one time. In many cases, it is desirable that two calls be simultaneously handled at the s ecial or executive substation. Due to the ease of operation and convenience in using, it is also f equently desirable to provide, at a substation of the character mentioned. auxiliary transmitting means and auxiliary receiving means, including an amplifier and a loud speaker, which auxiliary transmitting and receiving means are arranged to be used in lieu of the regular substation transmitier and receiver and may be conditioned for operation and used without removing a hand set or receiver from an associated supporting hook or cradle. It is further desirable to be able to ronnect and arrange the auxiliary transmitting and receiving means so that they may operatively be included in either of two established connections, without releasing either of the two connections.

It is an object of the present invention to provide, in a telephone system, improved apparatus of the general character briefly described above which is so arranged that two communication connections may simultaneously and easily be established between a special substation and two different lines of the system and in which either of two sets of transmitting and receiving means may be used in communicating over either of the two established connections.

It is a further object of the invention to provide in an arrangement of the character described, improved apparatus for selectively connecting one set of transmitting and receiving means to be used in communicating over either of two established connections respectively extending between the substation at which the transmitting and receiving means are located and two different lines of the system.

It is a still further object of the invention to provide, in an arrangement of the character described, a substation comprising auxiliary transmitting means and auxiliary receiving means including a signal amplifier, together with improved apparatus for permitting the auxiliary transmitting and receiving means to be used to the fullest extent on calls incoming to and outgoing from the substation.

The invention is illustrated as being incorporated in an automatic telephone system including an exchange at which the plurality of lines of the system terminate. One of the lines of the system extends to a special or executive substation of the character mentioned above. Automatic switching equipment of the impulse responsive type is provided in the exchange for establishing connections between the lines extending to the exchange, the arrangement being such that the automatic switching equipment is controllable from the executive substation or by way of any of the lines extending to the exchange, to establish communication connections between the executive substation and the lines. There is also provided in the exchange, special automatic switching means having access only to the group of subordinate lines to which calls ar frequently initiated at the executive substation. This special automatic switching means is individual to the executive substation and is controllable only from this substation to establish a second communication connection between the substation and a second of the lines extending to the exchange. This second communication connection may be set up in the absence of a connection of the first type as mentioned above, or may be set up while a connection of the first type exists and without releasing the same. More specifically. there is provided, at the executive substation, a plurality of key operated calling switches which individually correspond to the subordinate lines forming the group accessible to the automatic switching means mentioned above, and control ipparatus operative in response to the operation of any one of the calling switches for causing the special switching means to extend a connection from the executive substation to the particular one of the subordinate lines corresponding to the operated calling switch. For the purpose of communicating over a connection of the first type, there is provided, at the special or executive substation, the usual transmitter and receiver, which two elements are normally connected and arranged to be used in conversing over a connection of the first type. Auxiliary transmitting and receiving means are also provided at the special substation which are normally connected and arranged to be used in communicating over a connection of the second type. The auxiliary receiving means comprises a. signal amplifier and a loud speaker, the input circuit of the signal amplifier being interconnected with the auxiliary transmitting means and an anti-side-tone impedance network to form a substation circuit arranged for side tone suppression. The arrangement is such that the signal amplifier is conditioned for operation in response to the operation of any one of the calling switches to a position corresponding to one of the called subordinate lines.

Three diflierent embodiments of the invention are disclosed, in all of which means are provided for operatively connecting the auxiliary transmitting and receiving means to be used in communicating over either of the two type of connections briefly described above. More particularly, a pair of relays are provided which are under the control of manually operable answer and transfer switches located at the substation and are operative selectively to connect the auxiliary transmitting and receiving means to be used in communicating over an established connection of either type. The arrangement is such that the two relays are jointly operative to transfer the auxiliary transmitting and receiving means from a connection of one type of a connection of the other type, and one of the two relays is operative to condition the signal amplifier provided in the auxiliary receiving means for operation. The answer switch includes contacts for completing the operating circuits of both relays, and the transfer key switch includes contacts for interrupting the operating circuit of the transfer relay without deenergizing the other relay, thus providing an arrangement wherein the desired transfer of the auxiliary transmitting and receiving means may be effected by selective operation of the answer and transfer switches. In one embodiment of the invention the regular substation transmitter and receiver can only be used in communicating over a connection of the first type, namely a connection established in response to the operation of the calling device provided at the executive substation or a connection established on a call incoming to the substation. In the second embodiment ofthe invention, a manually operable switch is provided at the executive substation, together with means operative in response to the operation of the switch for connecting the regular substation transmitter and receiver to be used in communicating over a connection of the second type, namely, a di e call connection. In the third embodiment of the invention, there is provided a relay which is operative in response to the removal of the substation receiver from its supporting hook or cradle to connect this receiver to be used in communicating over a connection of the second type, namely, a direct call connection. In accordance with a more specific feature of the invention, the above-mentioned relay is provided with a holding circuit which is arranged to be completed in response to the operation of the relay and serves to deliver energizing current to the substation transmitter following its completion. This relay also includes a set of contact springs for preventing alternating current energy from being delivered to the input circuit of the auxiliary receiving means when the relay occupies its operated position.

The arrangement of the apparatus is such that an established connection, extending between the special substation and one of' the subordinate substations and including the special switching means mentioned above, is only locally interrupted at the subordinate substation in response to the usual release operation performed thereat, the switching equipment involved in the connection being held operated until a release operation is performed at the special substation. In order to prevent a subordinate from hanging up and then reestablishing the connection for the purpose of listening in on a conversation ensuing at the executive or special substation, there is provided in the special switching means, apparatus responsive to a release operation performed at the subordinate substation for rendering the established connection ineffective to transmit signal frequency currents from the executive substation to the subordinate substation.

Further features of the invention pertain to the particular arrangement of the apparatus whereby the above and additional operating features are attained.

The novel features believed to be characteristic of the invention are set forth with particularity of the appended claims. The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the specification, taken in connection with the accompanying drawings, in which Figures 1 to 6, inclusive, taken together, illustrate a telephone system having incorporated therein the above outlined features of the present invention; Figs. 6a and 6b are modifications of the apparatus shown in Fig. 6; and Fig. 7 is a diagram illustrating the manner of assembling the other figures of the drawings. More specifically, Figs. 1 and 6 illustrate the apparatus provided at one of the master telephone stations of the system. Figs. 3 to 5, inclusive, illustrate the circuit arrangement of the link circuit and switching apparatus individual to the telephone station shown in Figs. 1 and 6, and Fig. 2 illustrates the line selecting relay equipment through operation of which the automatic switching apparatus shown in Figs. 3 to 5, inclusive, is controlled to select a desired called line.

Referring now more particularly to Figs. 1 to 6 of the drawings, the equipment there illustrated is, with the exception of the substation apparatus shown in Fig. 1 and the substation apparatus 600 shown in Fig. 6, included in an automatic exchange of the type frequently used to provide telephone facilities between the offices of a large manufacturing organization. This type of exchange is commonly known as a private automatic exchange. Extending to the exchange and terminating in automatic switching equipment diagrammatically illustrated at 320, are a plurality of substation lines, three of which are indicated at 32L 322 and 323. The ends of the lines remote from the exchange terminate in substations of the automatic type, that associated with the line 32l being diagrammatically illustrated at B and that associated with the line 322 being indicated at A. The line 323 extends to a special or executive substation of the character mentioned above and includes two line conductors C321 and C328 terminating in the telephone substation apparatus diagrammatically illustrated at 500. The automatic equipment 320 comprises the usual subscriber-controlled automatic switches for setting up connections between the telephone lines of the system. More particularly, these switches are, in accordance with conventional practice, divided into two classes, namely, nonnumerical switches, such, for example, as line finder or line switches, and numerical switches, such, for example, as selector and connector switches, which latter switches are responsive to impulses transmitted thereto over calling ones of the telephone lines and function to perform the usual line group, subgroup and unit selecting operations. The connector switches at which the lines terminate are, in accordance with conventional practice, equipped to perform the usual auxiliary functions of: testing a selected called line to determine the idle or busy condition thereof, returning busy tone over the calling line in the event a selected called line is found busy, and projecting ringing current over a selected called line to actuate the signal device at the substation in the event the called line is idle at the time it is tested.

The substation apparatus 600 and the apparatus provided at each of the substations A, B, etc., may be of any desired commercial construction and arrangement. In brief, this apparatus may comprise the usual hand set type of transmitter and receiver, a cradle or hook comprising supporting means for the receiver, a cradle or hook switch, a ringer, and an impulsing device for transmitting switch control impulses by way of the associated lines to the automatic switching equipment 320 located in the line terminating exchange. Preferably, the elements of each substation circuit are connected and arranged to form the well-known anti-side-tone circuit, whereby operation of the substation transmitter does not cause operation of the substation receiver.

As pointed out above, the present invention relates to the provision of improved apparatus for increasing the utility of the available telephone facilities at the executive substation where the substation apparatus 600 is located. With the apparatus thus far described. communication connections may be established between the executive substation and any of the other substations associated with lines terminating at the automatic switching equipment 320 in the usual manner. More particularly, on regular calls initiated at the executive substation, the calling device or dial operated impulsing device included in the apparatus 600 is actuated in the usual manner causing impulse trains to be transmitted to the automatic switching eqipment 320. This equipment responds to the impulse trains by routing the connection to the desired called line. All calls incoming to the executive substation are routed by way of the regular automatic switching equipment 320 provided in the exchange and the line 323 to the substation apparatus 600, the signal device or ringer embodied in this apparatus being energized in response to ringing cur rent projected over the line 323 following seizure of the line by one of the connector switches included in the automatic switching equipment 320.

For the purpose of routing calls from the executive substation directly to certain of the frequently called subordinate lines terminating at the automatic switching equipment 320 without operating a calling device of the type conventionally used in automatic systems and without the delay incident to the operations required to set up a connection in a manual system, there is provided automatic switching means comprising a rotary switch 3|0 having bank contacts at which the group of subordinate lines terminate. This switch is a standard twenty-five point rotary switch and comprises four sets of bank contacts 3 to 3M, inclusive, wipers M5 to 3l8, inclusive. respectively associated with the enumerated contact sets, and means comprising a motor magnet 3l9 for driving the enumerated wipers over the contacts of their respective associated sets. As indicated, the contacts in the two sets 3l3 and 3M terminate the line conductors of the respective subordinate lines which are directly accessible to the executive substation. and the contacts in the set 312 terminate the private or control conductors of the subordinate lines. The contacts of the fourth contact set 3 terminate conductors extending by way of the cable 295 to the contact springs of a group of coding relays shown in Fig. 2. These conductors individually correspond to the subordinate lines and are included in marking circuits by means of which operation of the switch 3| 0 is controlled. Since the control apparatus described hereinafter is only arranged to provide direct calling facilities to twenty subordinate lines only twenty of the contacts of each contact set em odied in the switch (H0 are used. the remaining contacts being left blank. The switch 3| 0 is arranged to be controlled bv the group of coding relays shown in Fig. 2 and by the control apparatus 415 iilustrated in Figs. 4 and 5. More particularly, the coding relays shown in Fig. 2 and indicated at R200, R2l0, R220, R230, R240, R250 and R260. respectively, are arranged to be energized in combinations of three in response to the selective operation of any one of the plurality of calling switches illustrated in Fig. 1. These switches, which are indicated at I00 to I09, respectively, individually correspond to the lines forming the group of subordinate lines directly accessible to the executive substation. Each of the enumerated calling switches is of the key operated type and is operable from a normal position to either of two off-normal positions individually corresponding to two subordinate lines. Each of the switches is of the well-known locking type, such that, when once operated to an off-normal position, it remains in operated position until manually restored to normal. The contact springs of the calling switches are wired in the manner illustrated so that, when any one thereof is operated to one of its two oiT-normal positions, three of the coding relays shown in Fig. 2 are caused to operate in the manner described in detail hereinafter to complete one of the above-mentioned marking paths extending to one of the contacts in the set 3H of the rotary switch 3l0,

thereby to identify the called line desired by the calling party.

The line wipers 3 I 1 and 3 I 8 of the rotary switch 3I0 are arranged to be connected by way of a link line comprising the conductors C335 and C339 to a repeating transformer 500 through operation of the control apparatus indicated generally at MS. This apparatus comprises a start relay R450 which is arranged to be energized in response to any one of the calling switches shown in Fig. l and functions to initiate the operation of the rotary switch 310 to select a desired called line. Operation of the rotary switch is controlled by the three relay R420, R430 and R440, of which the relays R430 and R440 directly control the stepping operation of the switch 3H) and the relay R420 is a test relay for arresting the operation of the rotary switch when its wipers are operated to a position corresponding to a desired called line. The relay equipment also includes a pair of relays R400 and RAID which function to test the condition of a selected called line to determine whether it is idle or busy. More particularly, the slow-torelease relay R4I0 is a busy relay which is operative to return busy tone to the executive substation in the event a selected called line is found busy, and the slow-to-operate relay R400 is operative in the event a selected called line is found idle and is arranged to control the ringing relay R560, shown in Fig. 5. The busy relay R4l0 also functions as a device for impressing a background signal on an established connection involving a busy subordinate line called from the executive substation. The apparatus 4I5 also includes a ring cutoff relay R550 and a back bridge or called party answer relay R5l0, which relays are arranged to perform the functions conventionally required of relays of this character.

As indicated above, auxiliary transmitting and receiving means are provided at the executive substation for use in communicating over connections established through operation of the auxiliary or special switching means just described. The auxiliary transmitting means comprises a microphone I35, and the auxiliary receiving means comprises a sound reproducing device in the form of a loud speaker, diagrammatically illustrated at I36, connected in tandem with a signal amplifier including the vacuum tube 620. The auxiliary transmitting and receiving means are interconnected with an anti-side-tone impedance network indicated generally at 630 to form a substation circuit of the well-known antiside-tone type having line terminals 643 and 644 arranged to be connected by way of a link line including the conductors C515 and C516 to one winding of the repeating transformer 500 or by way of a second link line comprising the conductors C295 and C291 to one winding of a second repeating transformer 300, shown in Fig. 3. More particularly, the anti-side-tone impedance network 630 comprises an induction coil or transformer IiIIl including five windings BII to GIS, inclusive, the last-mentioned of which is coupled to the primary winding SIB of an input transformer 5I8 having its secondary winding 6I1 connected across the non-adjustable terminals of a voltage dividing resistor 6I3. The adjustable terminals of the resistor 6I9 are coupled to the control grid MI and the cathode 622 of the tube 620.. The tube 620, which is of the well-known pentode type and includes a directly heated cathode, is provided with an output circuit comprising the primary winding 528 of an output transformer 621 connected in series with a source of anode potential between the anode I523 and the cathode 622. For the purpose of biasing the control grid 62I negatively with respect to the cathode 622, there is provided a resistor 525 which is commonly included in the output circuit of the tube and the energizing circuit for the cathode 622. The anode current source is by-passed for signal frequency currents by a condenser 626. The secondary winding 629 of the transformer 621 is connected to impress the signal voltage developed thereacross upon the input terminals of the loud speaker I36 through a volume control network I4I. This network includes three resistors I42, I43 and I44 and an adjustable contact carrying arm I45 so arranged that adjustment of the arm I45 varies the magniture of the signal voltage impressed upon the input terminals of the loud speaker I36 without substantially changing the value of the resistance connected in shunt with the transformer sec ondary winding 629. By this arrangement, ad justment of the signal input level to the loud speaker I36 may be achieved, in a well-known manner, without changing the output impedance into which the tube 620 operates and, hence, without introducing distortion into the reproduced output of the loud speaker I36.

The anti-side-tone impedance network 630 includes, in addition to the transformer 6 I0. a pair of line balancing circuits which are arranged selectively to be operatively included in the network and are individually effective, under different operating conditions, to balance the circuit so that, during operation of the microphone I35, no substantial portion of the developed signal voltage is impressed on the input circuit of the amplifier tube 520. The first of the two balancing network comprises an adjustable inductance element E3I connected in series with an adjustable resistor 632, and the second network similarly comprises an adjustable inductance element 63I' connected in series with an adjustable resistor 632'. As shown, the transmitter microphone I35 is included in a substantially closed transmission circuit comprising this microphone, a low impedance condenser I39, and the primary winding I38 of a coupling transformer I31. This transformer is provided with a secondary winding I40, one terminal of which is connected by way of the conductor CH6 to the junction point between the two transformer windings SH and 6 I2 and the other terminal of which is connected by way of the conductor CI41 to the junction point between the two transformer windings BI3 and GM. The two junction points noted form the conjugate terminals of the anti-side-tone bridge circuit which is effectively formed when the auxiliary substation equipment is included in an established connection. More particularly, signal currents traversing the secondary winding I40 of the transformer I31 during operation of the microphone I35 divide at the two junction points noted, portions of the currents traversin a path comprising the two windings 6H and 6I3 and one of the two balancing circuits, described above, and other portions of the currents traversing the two windings 612 and SM and the established loop circuit extending to the repeating transformer 500 or the repeating transformer 300. By suitably proportioning the impedance values of the elements respectively included in the two balancing circuits, the currents traversing the windings BI3 and BM are rendered substantially equal in magnitude and opposite in phase at all frequencies within the audio range irrespective of which of the two repeating transformers 300 and 500 is included in the established connection. Similarly, when the impedance values of the elements included in the balancing circuits are proportioned in the manner indicated, the signal currents respectively traversing the two windings 6H and EI2 are rendered substantially equal in magnitude and opposite in phase at all frequencies within the audio range. Thus, during operation of the transmitter microphone I35, the currents traversing the windings GII, H2, H3 and 6M cause substantially equal and opposite signal voltages to be induced in the secondary winding 6I5, so that substantially no signal energy is delivered to the input circuit of the vacuum tube 620. By virtue of this arrangement, suppression of the side tone is achieved.

For the purpose of utilizing the auxiliary transmitting and receiving means to answer calls incoming to the executive substation and routed by way of the substation line 323, and for the further purpose of permitting the auxiliary transmitting and receiving means selectively to be connected for use in communicating over either of two simultaneously established connections, each extendin to the executive substation, there are provided an answer switch III, a, transfer switch 0, the repeating transformer 300, the link line comprising the two conductors C295 and C291, and the two control relays R210 and R520, The key operated answer switch I I I is of the well known locking type and the key operated transfer switch IIO is of the non-locking type. Both of the two relays R210 and R520 are arranged to be energized in response to the operation of the answer switch III, the first thereof being operative to condition the signal amplifier comprising the tube 620 for operation, and also to complete an auxiliary loop circuit extending to a distant subordinate substation and terminated at the windings 303 and 304 of the repeating transformer 300. Arranged to be serially included in an established auxiliary loop circuit is an adjustable resistor 306 for conducting direct current and bypassed for signal frequency currents by a condenser 305. The relay R210 conditions the signal amplifier for operation by supplying anode and cathode energizing voltages to the tube 620 over circuits traced hereinafter. In this regard, it is pointed out that the tube 620 is of the wellknown C33 type, which type of tube requires only a low anode voltage to provide reasonably good amplification. Hence, the exchange battery is used to supply anode and screen voltages to this tube. The exchange battery also supplies energizing current to the cathode 622 of the tube, a

voltage dropping resistor 624 being serially included in the cathode energizing circuit for the purpose of fixing the resistance of the circuit so that the rated current of the cathode heater traverses the circuit.

The relay R520, which is energized in response to operation of the answer switch II I and deenergized in response to the subsequent operation of the transfer switch I I0. is arranged to transfer the link line C515. C516, leading to the input terminals 643 and 544, from a connection with the winding 50I of the repeating transformer 500 to a connection with the winding I of the repeating transformer 300. This relay also functions selectively to include the two balancing circuits, described previously, in the antl-side-tone impedance network 530 in the manner pointed out more in detail hereinafter. In order to remind a user of the executive substation equipment that the answer switch III is in its operated or ofi-normal position, there is provided a signal lamp I25 which is arranged to be energized in response to the operation of the switch III to its off-normal position.

For the purpose of preventing an established connection extending to one of the subordinate substations from being reestablished after the usual release operation is performed at the subordinate substation and before the connection is released at the executive substation, there are included in the control apparatus 4i 5 two relays R530 and D540, the first of which is of the slowto-operat'e type, and the last-mentioned of which is of the slow-to-release type. More particularly, current for energizing the transmitter microphone I35 is normally supplied to this microphone over a circuit including the winding of R540. When, however, a release operation is performed at a subordinate substation involved in an established connection extending to the executive substation, the relay R540 cooperates with the back bridge relay R5I0 to cause the operation of the relay R530, and the last-mentioned relay functions to open the circuit for supplying energizing current to the transmitter microphone I35 and to maintain this circuit open so long as the connection is not released at the executive substation.

Although the apparatus shown in detail in Figs. 1 to 6, inclusive, may be arranged in any desired manner, it is contemplated that only the apparatus shown in Fig. 1 and the substation apparatus 600 shall be located in the oflice of the executive for whose use it is provided. It is further contemplated that the remaining apparatus illustrated in the drawings shall be conveniently located in the exchange terminating the various lines of the system, a suitable multi-conductor cable being provided for interconnecting the circuit elements to provide the circuit arrangement depicted in the drawings and described above.

Referring now more particularly to the operation of the special switching equipment to establish a connection to one of the regular automatic substations and assuming that the substation A is being called, the call is initiated by operating the calling switch I00 to the particular off-normal position corresponding to the called substation. More particularly, if the substation A is being called, the key switch I00 is operated from its normal position to its lower off-normal position to close the springs of the three sets H4, H5 and H6. As indicated above, three of the coding relays illustrated in Fig. 2 respond to the operation of any one of the key switches to an ofi-normal position to provide a marking path for marking the particular called line in the contact level 3II of the rotary switch 3I0. In the case under consideration, when the springs H4, H5 and H5 are closed, circuits are completed for energizing the three relays R200, R2I0 and R220, which circuits commonly include the winding of the start relay R450. The operating circuits for the three relays noted may be considered as a single circuit extending from battery at the lower terminal of the winding of R450 by way of this winding to C203, where it divides; one branch extending by way of the winding of R220, CI 32 and the springs M4 to the grounded conductor Cl21; a second branch extending by way of R2I0. CI33 and the springs H5 to the grounded conductor CI21; and

the third branch extending by way of R200, C134 and the spring 115 to the grounded conductor C121. When energized over the above-traced circuits, the four relays R200, R210, R220 and R450 operate and the first three thereof function to complete a marking path identifying the line 322 as the line being called. This marking path extends from ground at RA201 by way of RA211. RA221, and C211 to the twenty-fifth contact of the contact set 311. Due to the above-described arrangement of the coding relays, none of the other marking paths individually corresponding to the other lines directly accessible to the special equipment are completed.

As a further example of the manner in which the coding relays operate in response to selective operation of the calling switches, it is pointed out that if the switch 100 is operated to close the contact springs 112, 113 and 111 a series-parallel circuit is completed for energizing the three coding relays R200, R210 and R230, and the start relay R450. In this case the two relays R200 and R210 are energized over circuits substantially similar to the branch circuits traced above and the relay R230 is energized over a branch circuit extending by way of C293, the winding of R230, C131 and the contact spring 111 to the grounded conductor C121. When the three relays R200, R210 and R236 operate, a marking pathis completed for applying ground potential to the twenty-fourth contact of the set 311, this path extending from ground at RA201 by way of RA21 1, RA231 and C218 to the twenty-fourth contact of the set 31 1.

Again, if the tenth calling switch 100 is operated to its lower off-normal position, the springs 121, 122 and 123 are closed to complete a seriesparallel circuit for energizing the three relays R200, R240 and R250 in series with the relay R450. In this case the relay R200 is energized over a branch circuit extending by way of C134 and the springs 123 to the grounded conductor C121; the relay R240 is energized over a branch circuit extending by way of C130 and the springs 122 to the grounded conductor C121; and the relay R260 is energized over a branch circuit extending by way of C128 and the springs 121 to the grounded conductor C121. Upon operating, the three relays complete a path extending from ground at RA261 and by way of RA246, RA203 and C219 for impressing marking ground on the nineteenth contact on the contact set 31 1. It will be apparent from the foregoing description that by selectively operating the calling switches, the coding relays may be caused to operate in diderent combinations of three, thereby selectively to complete the twenty marking paths extending to the respective contacts of the contact set 311.

In any case when three of the coding relays are energized the start relay R450 is also energized and, upon operating, completes, at RA452 and RA453, respectively, paths commonly including the grounded armature RA559 and C410 for applying ground to the tone apparatus and ringing machine start conductors C401 and C408, thereby to initiate the operation of the tone apparatus and the ringing apparatus in the usual manner. At RA454, the relay R450 applies ground to the amplifier start conductor C294. thereby to condition the amplifier comprising the tube 620 for operation. More particularly, operation of the relay R450 results in the connection of the positive terminal of the exchange battery to C254, so that anode potential is impressed through the primary winding 628 of the output transformer 621 on the anode 623 of the vacuum tube 620. Operation of the relay R450 also results in the completion of a circuit for energizing the cathode 622 of the tube 620, this circuit extending from the grounded positive terminal of the exchange battery at RA454 by way of C294, the voltage dropping resistor 624, the cathode 622 and the biasing resistor 525 to the negative terminal of the exchange battery. It will be noted that positive screen potential is applied to the screen electrode of the tube 620 directly from the positive terminal of the exchange battery over the path including the conductor C294.

At RA451, the relay R450 completes a circuit for energizing the slow-to-operate relay R430, this circuit extending from ground at RA451 by way of RA422, RC423, C336, the armature 310'. C331 and the winding of R430 to battery. The relay 430 operates, after an interval, and, at RA431, prepares a circuit, traced hereinafter, for energizing the relay R420 when the wipers of the rotary switch 310 are stepped to the position corresponding to the selected called line 322. At RA432 and RC433, the relay R430 also prepares a test circuit, traced hereinafter, for energizing the busy relay R410 in the event the selected called line is busy at the time the switch 310 is operated to the position corresponding to the called line. At RA435, the relay R430 completes a circuit for energizing R440, this circuit extending from ground at RA451 by way of RA422, RC423, C335, the armature 31!, C331, RA435 and the winding of R440 to battery.

When energized over the circuit just traced,

the relay R440 operates to complete, at RA441, a circuit for energizing the motor magnet 319 of the switch 310, this circuit extending from ground at the grounded contact R0421 by way of RA426, RA441, C338, and the winding of the magnet 319 to battery. Upon operating, the magnet 313 steps the wipers 315 to 318, inclusive, of the switch 310 one step and interrupts, at its associated armature 319, the above-traced operating circuits for the two relays R430 and R440. The relay R440 immediately restores to interrupt, at RA441, the operating circuit as traced above for the magnet 319, whereupon this magnet restores its associated armature 319' to recomplete the operating circuits for the two relays R430 and R440, re spectively. When the operating circuit for R440 is again completed, this relay operates to recomplete the above-traced circuit for energizing the magnet 315, causing this magnet to again step the enumerated wipers of the switch 310 a second step and to again interrupt the operating circuits for the two relays R430 and R440. The abovedescribed interaction between the relay R440 and the magnet 315 continues until such time as the wipers of the switch 310 are stepped to the position corresponding to the called line 322, or until the wiper 315 is moved into engagement with the contact marked with ground over the abovetraced marking path. During the interaction between the relay R440 and the magnet 319, the relay R430, due to its slow-to-release characteristic, stays operated.

When the wiper 315 of the switch 310 is operated into engagement with its associated twentyfifth contact, the above-mentioned circuit is completed for energizing the relay R420, this circuit extending from the grounded conductor C211 by way of the twenty-fifth contact of the set 31 I, the wiper 315, C333, RA431 and the lower winding of R420 to battery. When its lower winding is energized over the circuit, just traced, the relay R420 immediately operates to interrupt, at RA42G and RC421, the above-traced circuit for energizing the magnet 319, thereby to arrest the interaction between this magnet and the relay R440 and to prevent further movement of the wipers embodied in the rotary switch 310. At RA425, the relay R420 completes a holding circuit for itself, this holding circuit extending from the grounded conductor C333, by way of RA425 and the upper winding of R420 to battery. At RA422 and R0423, the relay R420 interrupts a point in the circuits over which R430 and R440 are being held energized, causing these two relays to restore. At RA422 and R0424, the relay R420 prepares a holding circuit, traced hereinafter, for R560.

.At RA421, the relay R420 completes the abovementioned circuit for testing the called line 322 to determine the idle or busy condition thereof. In this regard it is pointed out that the automatic switching equipment provided in the exchange 320 is conventionally arranged so that, when the line 322 is busy, ground potential is present upon the control conductor C324 thereof, whereas, if the line is idle, battery potential appears on this control conductor. line 322-is busy, the busy relay R410 is energized over the above-mentioned test circuit, which circuit extends by way of the control conductor C324, the twenty-fifth contact of the set 312, the wiper 316, G334, RA421, RA432, R0433 and the winding of R410 to battery. Upon operating, the busy relay R410 interrupts, at RAM 1, a point in the circuit, traced hereinafter, for energizing the slow-to-operate control relay R400. At RA412, the relay R410 completes a holding circuit for itself, this circuit extending by way of the conductor C334, RA421, RA4I2 and the winding of R410 to battery.

The relay R410, upon operating, functions to complete a circuit for impressing busy tone voltage on the input circuit of the signal amplifier comprising the vacuum tube 620. Specifically, the path of the busy tone current extends from the ungrounded terminal of the busy tone source, not shown, by way of the busy tone conductor C406, the condenser 404, RA4I3, C465, R0529, RA528, C518, the transformer windings 812 and 61 I, the inductance element 631, the resistor 632, G512, R6522, RA521, C514, the transformer windings B13 and E14, C515, RA524, RC525 and the upper half of the Winding 501 to the grounded terminal of the busy tone source at the grounded center tap 502 of the winding 501. This current causes an induced voltage to be developed in the transformer winding 515 which is impressed through the coupling transformer 618 between the input electrodes of the tube 620, and the amplified output energy is delivered through the transformer 621 and the volume control network 141 to the loud speaker 136 for reproduction. The reproduced busy tone signal indicates to the calling party that the desired connection can not immediately be established.

Presumably, the substation at which the illustrated special equipment is located is that of an important official or executive, such that calls originating at this substation and intended for the various subordinate lines are of sufficient importance to warrant direct attention on the part of the called parties. For this reason, the busy relay R410 is so arranged that it functions to impress a background signal on a selected subordinate line which is tested and found busy. More particularly, when the relay R410 operates, it completes a path for impressing the voltage of Assuming the selected called the ringing current source upon the previously established loop circuit of which the followed line forms a part. In the case under consideration, the path of the ringing current extends from the ungrounded terminal of the ringing current source, not shown, by way of the ringing current conductor C405, the condenser 402, RA4I4, C339. the wiper 318, the twenty-fifth contact of the set 314, the line conductor C326, the receiver at the called substation A, the line conductor C325, and through one winding of the back bridge relay conventionally provided in the operated connector switch of the switch train leading to the line 322, to the grounded terminal of the ringing current source. The condenser 402 included in this path is selected of a value such that the cur rent traversing the abovetraced path is unsufficient to produce a signal which will interfere with the ensuing conversation over the established connection including the line 322, but is of sufficient strength to indicate that the substation A is being called from the executive substation.

Following the operation of the busy relay R410, no further operation of the apparatus occurs until the partially established connection is released. To release the connection it is only necessary to restore the operated key switch to its normal position, thereby to interrupt the abovetraced operating circuits for the four relays R2 00, R2I0, R220 and R450, causing all of these relays to restore. When the first three of the enumerated four relays restore, the above-traced marking path extending to the twenty-fifth contact of the contact set 311 in the switch 310 is interrupted. When this path is interrupted, the holding circuit for R420 is broken, causing this relay to restore to interrupt, at RA421, a point in the abovetraced holding circuit for the busy relay R410. Thus, the three operated relays R410, R420 and R450 are caused to restore and the common apparatus 415 is conditioned for further use. In this regard, it will be noted that when the relay R450, restores, it interrupts, at RA454, the path for applying ground to the amplifier start conductor C294, whereby the signal amplifier including the tube 620 is rendered inactive. Upon restoring, the relay R450 also interrupts, at RA451, a point in the operating circuits for R430 and R440 and, at RA452 and RA453, the two paths described above for applying ground to the tone apparatus start conductor C401 and the ringing apparatus start conductor C408.

If the calling party desires he may disregard the busy signal and wait until the established connection is cleared out, in which case the apparatus 415 operates to extend the connection to the called line 322 immediately the called line becomes idle. In this regard it is pointed out that the abovetraced holding circuit for the busy relay R410 extends to ground as applied to the control conductor C324. Hence, when the line 322 becomes idle and ground is removed from C324, the holding circuit for R410 is interrupted causing this relay to restore. Upon restoring, the busy relay R410 permits ringing current to be projected over the called line in the manner described immediately hereinafter.

In the event the called line 322 is idle at the time it is selected by the switch 310, battery potential is present on the control conductor C324 thereof and, hence, the busy relay R410 is not energized. A short time interval after the switch 310 is operated to the position corresponding to the line 322 or, more particularly, a short time interval after the relay R420 operates, the

slow-to-release relay R430 restores to interrupt, at RA432 and RC433, a point in the previously traced testing circuit by way of which R4I0 is energized. At RA432 and R0434, the relay R430, upon restoring, completes a circuit for energizing the control relay R400, this circuit extending from ground at RA45I by way of RA4II, the winding of R400, RC434, RA432, RA42I, C334, the wiper 3I6 and its associated twenty-fifth contact to battery as impressed upon the control conductor C324 of the idle line 322. Shortly following its energization over the circuit just traced, the slow-to-operate relay R400 operates to complete, at RA40I, a circuit for energizing the ringing relay R560, this circuit extending from ground at RA45I by way of RA422, RC424, RA40I, C403 and the winding of R550 to battery. Upon operating, the relay R560 completes, at RA561, a holding circuit for itself, which circuit extends from ground at RA45I by way of RA422, RC424, C462, RA561 and the winding of R000 to battery. At RA560, the relay R560, upon operating, applies ground to the control conductor C324 of the called line 322, thereby to mark this line as busy in the bank contacts of the connector switches to which it extends. Specifically, the above-mentioned marking path extends from ground at RA508 by way of C454, RA42I, C344, the wiper 3I6 and its associated twenty-fifth contact to the control conductor C324. At RA565 and its associated grounded contact R0556, the relay R500 prepares a locking circuit, traced hereinafter, for the slow-acting ring cut-off relay R550. At RA50I and RA552, the relay R560 completes a path for projecting ringing current over the called line 322, thereby to energize the signal device at this substation. Specifically, the path traversed by the ringing current extends from one terminal of the ring current source, not shown, by way of C405, C461, RA551, RA50I, C339, the wiper 3I0 and its associated twenty-fifth contact, the line conductor C325, the signal device at the called substation, the line conductor C325, the wiper 3I1, C335, RA563, RA502, RA552, RC553 and the lower winding of R550 to the other terminal of the ringing current source. The resulting operation of the signal device provided at the called substation indicates that this substation is being called. The relay R560 also comi pletes a path for transforming ring-back-tone energy to the input circuit of the signal amplifier tube 020, this path being substantially the same as that traced previously for the busytone current, but extending from the ring-tone source by way of C405, the condenser 403, C458, RA563, RA550 and RA542, rather than by way of C406, the condenser 404, RA4I3 and C465. The ring-back-tone energy delivered to the input circuit of the amplifier tube 620 is amplified by this tube and delivered to the loud speaker I35 for reproduction, thereby to indicate to the calling party that the desired called substation is being rung.

The desired conversational connection is completed when the call is answered at the called substation A. More particularly, when the receiver at the substation A is removed from its hook, a direct current bridge is placed across the line conductors C325 and C326 of the line 322 to complete a circuit substantially similar to that, traced previously and traversed by the ringing current, for energizing the lower winding of the ring cutoff relay R550. When its lower winding is energized, the relay R550 first closes the X" contacts associated with RA556, thereby to complete the above-mentioned holding circuit for itself, this circuit extending from ground at R0506 by way of RA565 and the two windings of R550 in series to battery. At RA552 and RA551 and their respective associated resting contacts, the relay R550 opens two points in the above-traced path for projecting ringing current over the selected called line 322 and, in so doing, opens two points in its own operating circuit. At these same armatures and their respective working contacts, the relay R550 extends the called substation loop circuit to the repeating transformer 500, thereby to complete a circuit including this loop circuit for energizing the back bridge or called party answer relay R5I0.

When the relay R5I0 operates, it completes, at RA5II and RC5I3, a circuit for energizing the slow-to-release relay R540, this circuit extending by way of RA532, C51I, the terminals 642 and 639, CI50, the transmitter microphone I35, the winding I38, CI5I, the terminals 540 and SM, C510 and through the winding of R540 to battery. Current for energizing the transmitter microphone I35 is conducted to the microphone over the circuit just traced. Upon operating, the relay R540 prepares, at RA54I, a circuit, traced hereinafter, for energizing R530. At RA542, the relay R540 opens a. further point in a path for conducting ring-back-tone current by way of the anti-side-tone impedance network 630 to the input circuit of the amplifier tube 020.

Following the operation of R540, the desired conversational circuit is completely established, talking battery being supplied to the loop extending to the called substation A through the windings of the back bridge relay R5I0, and talking battery being supplied to the transmitter microphone l35 over the circuit, traced above, for energizing the relay R540. Signal currents incoming to the special substation and traversing the windings 503 and 504 of the repeating transformer 500 cause inducted voltages to be developed in the winding 50I coupled to the two windings 503 and 504, whereby signal currents corresponding to the incoming signal currents are caused to flow through the anti-side-tone impedance network 630. More particularly, the path followed by currents traversing the winding 50I may be traced as extending from the lower terminal of this winding by way of R0529, RA528, C515, the two windings GI2 and ill, the inductance element 63I, the resistor 532, C512, RC522, RA52I, C514, the two windings H3 and H4, C515, RA524, RC525, back to the upper terminal of the winding 50L The resulting induced voltages developed in the winding 6l5 are impressed through the coupling transformer 6I0 and the voltage dividing resistor BIS between the input electrodes of the amplifier tube 020, thereby to cause corresponding amplified currents to fiow in the output circuit of this tube. Such currents cause induced voltages to be produced in the winding 629 of the output coupling transformer 621, which voltages are impressed through the potentiometer network I4I on the input terminals of the loud speaker I35 for reproduction.

Audio frequency currents developed by operation of the microphone I35 are caused to circulate in the closed low impedance transmission circuit comprising the primary winding I38 of the transformer I31 and the condenser I33. Such currents cause corresponding induced voltages to be developed in the secondary winding I40 of the transformer I31 which are impressed by means 79, TELEPHONY,

of the two conductors CI46 and CI41 between opposite terminals of the anti-side-tone impedance network 530. More particularly, signal currents traversing the winding I40 divide at the terminals of the anti-side-tone impedance network and follow the two paths, traced previously, whereby the well-known anti-side-tone effect is realized.

The release of the established communication connection is primarily under the control of the calling party. If the called party at the substation A restores his receiver to its hook before the calling key switch I is returned to normal, the established called party loop circuit extending to the repeating transformer 500 is interrupted, causing the back bridge relay RI0 to restore in the usual manner. Upon restoring, the relay R5I0 interrupts, at RA5I I and RC5I3, the abovetraced circuit for energizing R540 and, at RA5II and RC 5I2, it completes a circuit for energizing R530, the latter circuit extending by way of RA521, RA54I and the winding of R530 to battery. Upon operating, the relay R530 completes, at RA53I, a. holding circuit for itself, thi circuit extending from ground at RA555 by way of RA53I and the winding of R530 to battery. It is noted that the relay R530, although of the slow-to-operate type, operates to complete its holding circuit before the slow-to-release relay R540 restores to open the operating circuit for R530. At RA532, the relay R530, upon operating, opens a further point in the above-traced operating circuit for R540 and, in so doing, opens a further point in the circuit by way of which energizing current is delivered to the microphone I35. At RA534, the relay R530 applies ground by way of C410 to the two start conductors C401 and C408, thereby to reinitiate the operation of the tone apparatus and the ringing apparatus providing this apparatus is not already in operation. At

RA533, the relay R530 completes an alternative path including C469 for impressing busy-tone voltage upon the input circuit of the signal amplifier tube 620, thereby to cause busy tone to be reproduced by the loud speaker I 35. The busytone signal produced by operation of the loud speaker I36 indicates to the calling party that the key switch I00 has not been restored to normal.

The purpose of providing the relay R530, which operates in response to the release operation performed at the called substation, is that of positively preventing eavesdropping on the part of a subordinate, following the termination of a conversation between the subordinate and the calling executive. Thus, in th absence of this relay and its associated circuits, the called party at the substation A could, after hanging up, again remove his receiver from its hook to recomplete the talking connection, provided the operated key switch I00 was not restored to normal by the calling party. In this regard, it will be appreciated that the back bridge relay R5I0 would respond to the recompletion of the loop circuit to again complete the circuit for delivering energizing current to th transmitter I35, Obviously, with this connection reestablished, the subordinate could listen to any conversation being carried on in the office of the executive. With the arrangement illustrated, however, the relay R530. upon operating, is held operated independently of the back bridge relay R5I0 and, in its operated position, holds interrupted. at RA532, the abovetraced path by way of which nergizing current is delivered to the transmitter microphone I 35. Thus, the channel for transmitting signal currents outgoing from the special substation to the called substation is rendered completely inactive and eavesdropping on the part of a subordinate is positively prevented.

As indicated above, the common control apparatus M5 is released in response to th restoration of the operated key switch I00 to its normal position. When this switch is returned to normal, the three operated coding relays R200, R2I0 and R220, and the start relay R450 are all caused to restore in the manner pointed out above. When the three enumerated coding relays restore, the above-traced holding circuit for R420 is interrupted, causing the last-named relay to restore and interrupt, at RA422 and R0424, the circuit over which R550 is being held energized. During the restoration of the relay R560, the contact springs RA553 and R0564 ar moved into engagement before the loop circuit extending to the called substation is interrupted, providing the called party has not restored his receiver to its hook. When the enumerated springs are moved into engagement, the condenser 543 is connected through RAI between the line conductors C339 and C335. This condenser absorbs the voltage transient which occurs when the talking loop is interrupted at RA55I following the restoration of R560 completely to normal and thus prevents the transient from being reproduced as an objectionable click by the receiver at the called substation. When R5Ii0 falls back, it interrupts, at RA555 and R0556, a point in the holding circuit for the ring cutoff relay R550, whereby the lastmentioned relay is caused to restore. The relay R550 also opens RASGI a point in the path connecting the condenser 543 between the link line conductors C335 and C339. Upon restoring, the relay R550 opens, at RA555, a point in the holding circuit for R530. If R530 is operated, it restores to interrupt, at RA533, a point in the above-mentioned alternative path for delivering busy-tone energy to the signal amplifier tube 620. At RA534, the relay R530 removes multiple ground from C410. Following the restoration of R530, the common control apparatus M5 is entirely released.

As indicated above, th substation apparatus 500 is normally connected and arranged to be used in communicating over connections established between the executive substation and one of the other substations on calls incoming to the executive substation. The special substation apparatus illustrated in Figs. 1 to 6, inclusive, is. however, so arranged that the auxiliary transmitting and receiving means comprising the microphone I35 and the loudspeaker I36 may, if desired, be used in lieu of the regular substation apparatus 600 to answer incoming calls. For example, if a call is initiated at the substation A and routed by way of the automatic switching equipment 320 and the line 323 to the substation apparatus 600 at a time when the line 423 is idle, ringing current is projected over the line to actuate the ringing device conventionally included in the apparatus 600, thereby to give an audible indication that the executive substation is being called. Obviously, the call may be answered in the usual manner by removing the receiver embodied in the substation apparatus 600 from its associated hook. Alternatively, the auxiliary transmitting and receiving means comprising the loudspeaker I36 and the transmitter microphone I35 may be used. In case the answering party elects to use the last-mentioned means, the answer switch III is operated to its oiT-norrnal position to clos the springs I25 and thereby complete a circuit for energizing the control relay R520. The circuit noted extends from the grounded conductor CI21 by way of the springs I25, the springs I24, I52, and the winding of R520 to battery. At the springs I25, a circuit is also completed for energizing the signal lamp I26 in series with the winding of R210, this circuit extending from the grounded conductor CI21 by way of the springs I25, the signal lamp I26, CI21' and the winding of R210 to battery. When energized over the above-traced circuit, the relay R210 operates and the signal lamp I26 is illuminated to indicate that the answer switch III occupies its off-normal position. The relay R210, upon operating, completes, at RA212, a path for applying ground to the amplifier start conductor C204, whereby the signal amplifier comprising the vacuum tube 620 is conditioned for operation in, the manner described previously. At RA 21 I, the relay R210 completes a direct current bridging path between the line conductors C321 and 0320 of the called line 323, which path includes the two windings 303 and 304 of the repeating transformer 300. More particularly, the bridging path noted extends from the line conductor C321 by way of the windings 304, the resistor 306, 0298, RA21I, C299 and the winding 303 to the opposite line conductor 0328. The operated connector switch by way of which the connection is routed to the line 323 responds to the completion of this path in the usual manner by interrupting the ringing operation.

As indicated above, the relay R520 is energized in response to the operation of the answer switch III to its ofi-normal position. When energized, this relay operates to complete, at RA528, the above-traced circuit for energizing R540 and by way of which energizing current is delivered to the transmitter microphone I35. At RA521, the relay R520 opens a point in the above-traced operating circuit for R530, thereby to prevent the last-mentioned relay from operating in response to the operation of R540. At RA524 and RA528 and their respective associated resting contacts R0525 and R0529, the relay R520 disconnects the line conductors C515 and 0516 from the winding 50I of the repeating coil 500, and, at these same armatures and their respective associated working contacts R0526 and R0529, connects the line conductors 0515 and C516 by way of the link line conductors 0296 and 0291, respectively, to the opposite terminals of the repeating transformer winding 30I. Thus, the repeating transformer 300 is incorporated in the newly established talking connection in lieu of the repeating transformer 500, which latter transformer is used on subordinate calls originated at the special substation. Since the impedance characteristics of the loop circuits respectively including the repeating transformers 500 and 300 may be different, it is desirable to provide alternative line balancing circuits arranged alternatively to be included in the antiside-tone impedance network 630. As described above, with the relay R520 deenergized, the line balancing circuit comprising the inductance element 63l and the resistor 632 is operatively included in the anti-side-tone impedance network 630. When, however, the relay R520 operates, it substitutes the 'other balancing circuit, namely, that comprising the inductance element 63I' and the resistor 632', for the first-mentioned balancing circuit in the anti-side-tone impedance network. This is accomplished through the operation of R520 to disconnect, at RA52I, the conductor C514, extending to the upper terminal of the winding 6|3, from theconductor C512 and to connect this conductor to the conductor 0513. It will be understood that the line balancing circuit comprising the inductance element 63I' and the resistor 632' is adjusted to have a frequency-impedance characteristic substantially matching that of the equivalent impedance of the repeating transformer 300 and the line impedances associated therewith as determined at the line terminals 643 and 644 of the impedance network 630. Similarly, the line balancing circuit comprising the inductance element 03I and the resistor 632 is adjusted to have a frequency-impedance characteristic substantially the same as that of the equivalent impedance of the repeating transformer 500 and the line impedances associated therewith as determined at the line terminals I543 and 644 of the network 630.

Following the operation of R520, the desired conversational connection is fully completed. After the connection is completed the answering party may, by removing the telephone receiver or hand set of the substation apparatus 600 from its supporting hook or cradle and restoring the operated answer switch III to normal, release the auxiliary transmitting and receiving means comprising the loudspeaker I36 and the transmitter I35 and continue the conversation by using the regular substation transmitter and receiver. Also, by restoring and removing the substation hand set from its supporting hook and operating the answer switch III between its normal and off-normal positions, the answering party may substitute one set of apparatus for the other as many times as desired during a single conversation. In any case, when the answer switch II I is restored to normal, the above-traced operating circuit for the relay R520 is interrupted to cause this relay and the relay R540 to restore in the order named, and the series circuit for energizing the signal lamp I26 and the relay R210 is interrupted causing the last-mentioned relay to restore. When R210 restores, it removes ground from the am plifier start conductor 0294, thereby to render the signal amplifier inactive, and opens, at RA21I, a point in the above-traced bridging path between the conductors C321 and 0328 of the line 323. Following the restoration of the three relays R520, R540 and R210, the auxiliary transmitting and receiving means are entirely disconnected from the line 323.

As pointed out previously, to initiate a call at the special substation intended for a substation connected to a line not directly accessible to the special switching means, it is necessary to employ the dial operated calling device conventionally included in the substation apparatus 600. Such a call is initiated in the usual manner by removing the hand set embodied in the apparatus 600 from its supporting cradle to cause a bridging path to be placed across the line conductors C321 and 0328 of the line 323. As a result of the completion of this path, one of the non-numerical switches included in the switching equipment 320 responds by extending the calling loop circuit through to one of the numerical impulse responsive switches of the exchange, whereby the seized switch is conditioned to respond to impulses transmitted thereto by way of the calling line 323. The seized impulse responsive switch and one or more additional impulse responsive switches successively respond to the impulses of successive digits dialed at the calling substation by extending the connection to the called line 32I in the usual manner. If, following the dialing operation, the selected called line is found busy, busy tone is returned over the calling loop and is reproduced by the receiver included in the apparatus 600 to give an audible indication that the desired connection can not be obtained. On the other hand, if the selected called line is idle, ringing current is projected thereover to actuate the signal device at the called substation and ring-back tone is conducted back over the calling loop circuit to indicate to the calling party that the called substation is being rung. When the call is answered at the called substation, the desired conversational connection is completed in the usual manner.

Following the completion of a connection to the substation B in the manner just described, the auxi iary transmitting and receiving means comprising the transmitter microphone I35 and loud speaker I36 may be substituted for the transmitter and receiver embodied in the hand set for use in transmitting and reproducing signal currents. Such substitution is accomplished in the exact manner pointed out above with respect to the calls incoming to the executive substation, that is, by operation of the answer switch III to its off-normal position to cause the operation of the relay R210 and the two relays R520 and R540 embodied in the common control apparatus M5. The three relays noted operate in the exact sequence and perform the same functions as are described in detail in a preceding portion of the specification. both during the substitution and during the release of the apparatus following the restoration of the answer switch III to normal. Here again, it will be appreciated that the regular substation transmitter and receiver may be substituted at will for the transmitting microphone I35 and the loud speaker I36 by operating the answer switch III and restoring or removing the hand set from its supporting hook in the proper order.

As indicated in the introductory portion of the specification, a calling party at the executive substation may utilize the special substation apparatus to initiate a direct call to one of the subordinate lines during th course of a, conversation over an established connection to another of the lines extending to the exchange housing the automatic switching equipment 320. If the ensuing conversation is being held over a connection utilizing the regular substation transmitter and receiver embodied in the substation apparatus 600, the second call to the subordinate line may be initiated in the exact manner described above. For example, if the calling party is using the regular substation apparatus 000 to converse over an established connection extending to the substation B and desires to secure information from the subordinate having an office in which the substation A is located. the direct call to the substation A is initiated by operating the calling switch I to the lower off-normal position in the manner described above. The coding relays R200, R2I0 and R220, the common equipment M5 and the rotary switch 3I0 respond to the operation of the switch I00 to its off-normay position by performing the previously described functions, whereby a talking connection including the loud speaker I36 and the transmitter microphone I35 is established between the executive substation and the substation A. This direct connection may be released in the manner set forth above.

If the loud speaker I36 and the transmitter I35 are being used as the receiving and transmitting means in an established connection extending to the substation B and it is desired to use the special substation apparatus to initiate a call to one of the subordinate lines, as, for example, the line 322, directly accessible to the special substation equipment. it is necessary to operate the calling switch I00 to the off-normal position corresponding to the line 322 and following this, to operate the transfer switch IIO to its off-normal position. When the calling switch I00 is operated to the off-normal position corresponding to the lin 322, the apparatus illustrated in Figs. 2, 3, 4 and 5 operates to cause a connection to be extended to the line 322 in the exact manner described above. In this regard it will be recalled that the three relays R210, R520 and R540 are operated. Hence, the amplifier comprising the vacuum tube 520 is conditioned for operation and the line conductors C515 and C516 incoming to the anti-side-tone impedance network 630 are connected through to the repeating coil 300. The operated condition of the three relays R210, R520 and R540 in no way interferes with the operation of the other relays included in the common control apparatus M5 to cause the connection to the desired subordinate line 322 to be established. When the transfer switch I I0 is operated to its off-normal position, following operation of the calling switch I00 to the offnormal position corresponding to the called subordinate line 322, the operating circuit for the relay R520 is interrupted at the springs I24, causing this relay to restore. When the relay R520 falls back, it disconnects the line conductors C515 and C516 from across the terminals of the transformer winding 30I and connects these line conductors to the respective terminals of the transformer winding 50I, thereby operatively to include the auxiliary transmitting and receiving means comprising the loud speaker I36 and the transmitter micro-phone I35 in the established connection extending to the called subordinate line 322. The relay R520, upon restoring, also disconnects the conductor C514 from the conductor C513 and connects it to the conductor C512, thus substituting the line balancing circuit comprising the inductance element 63I and the resistor 632 for the balancing circuit comprising the inductance element 63I' and the resistor 632 in the anti-side-tone impedance network 630. With the relay R520 in its restored position, the original circuit for supplying nergizing current to the microphone I35 and for holding R540 operated, is interrupted at RA528. At this time, however, the back bridge relay R5I0 is in its operated position so that a multiple circuit extending from ground at RA5II and by way of RC5I3 and RA532 is completed for maintaining R540 operated and for supplying energizing current to the microphone I35.

It will be noted that to effect the transfer operation just described the transfer switch H0 is operated to its off-normal position without restoring the answer switch III to its normal position. So long as the last-mentioned switch is retained in its ofi-normal position, the abovetraced circuit by way of which the relay R2I0 is energized is held completed and, so long as R210 is energized and remains in its operated position, the loop circuit extending to the substation B and including the line 322 is maintained completed at RA21I. After the two connections extending, respectively, to the substations A and B are established, the party at the executive substation may, by operating the transfer key between its normal and off-normal positions, converse over either of the two connections at will. Thus, it the party at the executive substation, after conversing with the called party at the substation A, desires to again converse with the party at the substation B, he may do so by returning the transfer switch IIO to its normal position to recomplete the above-traced operating circuit for R520. It will be apparent, in view of the foregoing description, that when R520 reoperates it substitutes the balancing circuit comprising the inductance element 63I and the resistor 632' for the balancing circuit 63I, 632 in the anti-side-tone impedance network, disconnects the line conductors C515 and C516 from the transformer winding 50I, and reconnects the line conductors C515 and C516 to the'terminals of the repeating transformer winding 30!.

In order to release the two established connections, it is only necessary to restore the operated switches I and III to normal. When the switch I00 is returned to its normal position, the three operated coding relays R200, R2 I 0 and R220 are caused to restore, together with the start relay R450. Following the restoration of these four relays, the remaining operated relays embodied in the common control apparatus 4I5 are caused sequentially to restore in the manner described in detail in a preceding portion of the specification. Also, when the answer switch III is returned to its normal position, the three operated relays R520, R540 and R210 are deenergized and restore in the manner described previously.

From the foregoing description it will be apparent that, if a party at the executive substation is conversing over an established connection routed by way of the switch 3H) and desires information from another party having a line terminating at the exchange housing the switching equipment 320, he may initiate a call to the substation of the second party without interfering with the already established connection by using the regular substation apparatus 600. The second call is forwarded to the desired substation by dialing the directory number of the called substation and through operation of the automatic switching equipment 320 in the exact manner described above. After the second connection is established, it will be apparent that the calling party at the executive substation may, by first operating the answer switch III to its off-normal position and then operating the transfer switch H0 from its normal position to its offnormal position and vice versa, cause the auxiliary transmitting and receiving means including the transmitter microphone I35 and the loud speaker I36 operatively to be included in either of the two established connections without releasing either connection.

With the apparatus illustrated in Figs. 1 to 6, inclusive, and described above, no provision is made for operatively including. the telephone transmitter and receiver embodied in the substation apparatus 600 in a connection which has been established in response to operation of one of the calling key switches shown in Fig. 1. Hence, on a direct line call, or a call resulting in a connection completed in response to operation of one of the calling switches, it is impossible for the calling party at the executive substation to maintain any secrecy with respect to the conversation held over the established connection. This is true for the reason that, with the arrangement described, the loud speaker I35 and the transmitter microphone I35 can only be used as the transmitting and receiving means in this type of connection.

In order to permit either of the two sets of transmitting and receiving means selectively to be connected for use in communicating over either of the two types of connections, the improved arrangement shown in Fig. 6a may be used in lieu of the apparatus shown in Fig. 6. More particularly, the apparatus shown in Fig. 6a, is so arranged that the transmitter and receiver embodied in the substation apparatus 600 may be operatively included in a direct call connection established as a consequence of operation of one of the calling switches shown in Fig, 1. In general, the arrangement of the apparatus illustrated in Fig. 6B is substantially similar to that shown in Fig. 6 and, accordingly, corresponding elements of the two figures have been identified by the same reference characters. For the purpose of substituting the transmitter and receiver embodied in the substation apparatus 600 for the transmitter microphone I35 and the loud speaker I36 in an established direct call connection, there is provided a key switch 655 of the locking type which includes contact springs 656 to 669, inclusive, wired in the manner shown. More particularly, the line conductors C321 and C328 of the substation line 323 are connected, respectively, to the springs B62 and 665 so that, when the switch 655 occupies its normal position, they are connected through to the substation apparatus 600. When, however, the key switch 655 is operated to its off-normal position, the two springs 663 and 666 are transferred from engagement with the springs 662 and 665, respectively, and into engagement with the springs 664 and 551, respectively, whereby the substation apparatus 600 is connected across the secondary winding 652 of a coupling transformer 650. The transformer 650 corresponds to the coupling transformer I31 associated with the closed transmission circuit including the microphone I35. The primary winding 65I of the transformer 550 is arranged to be connected across the opposite terminals of the anti-side-tone impedance network 630 in lieu of the winding I40 of the transformer I31 when the switch 655 is operated to its off-normal position. The switch 655 also includes contact springs for disconnecting the current feed conductor C510 from the transmission circuit including the microphone I35 and for connecting this conductor to supply energizing current to the transmitter included in the telephone substation apparatus 600. More particularly, with the switch 655 in its normal position, the conductor C510 is connected by way of the springs 551 and 650 to the conductor CI5I, whereby energizing current may be supplied to the transmitter microphone I35 over the circuit traced previously. When, however, the switch 655 is operated to its off-normal position, the spring 651 is moved from engagement with the spring 658 to open a point in the previously traced circuit, and is moved into engagement with the spring 656 to complete a circuit, traced hereinafter, for supplying energizing current to the transmitter embodied in the apparatus 600. For the purpose of rendering the loud speaker I36 completely inoperative to reproduce signal currents incoming to the executive substation at a time when the switch 655 is oper- 

